Stages are well known; they are typically used for precision motion where an object (for instance a workpiece) is carried on a stage. Stages are available which move in only one direction (i.e. x direction only), in two directions, (x, y) in three directions (x, y, z) or in six degrees of freedom. Such stages are used for instance in machine tools and lithography apparatus. Typically in lithography the stage carries an object which is a wafer or a reticle. The stage is supported by and moves in the x (or x and y) axis directions relative to a base structure which also supports other components of the system. This base structure is typically supported on a foundation by a spring/damper system to isolate the base structure and stage from outside vibrations. Such vibrations, if transmitted to the base structure, could cause relative position errors between the base structure and stage.
If the stage drive mechanism is mounted between the stage and base structure, the forces which accelerate and decelerate the stage also act upon the base structure equally and in the opposite direction. These reaction forces can cause unwanted motion and vibration in the base structure. This makes precision motion and positioning of the stage relative to the base much more difficult.
Hence it is known that is desirable to avoid effects of reaction forces applied to the base structure; see U.S. Pat. Nos. 4,525,659 and 5,172,160.
Present FIG. 1 shows the technical problem of reaction force to which the present invention is directed. The stage 10 (shown here in a side view) rides on bearings 12 on a flat surface 16 of a base structure 18. (It is to be understood that this drawing and the other drawings which form a portion of this disclosure are simplified to show only essential elements.)
The actual stage 10 is a fairly complex structure, including a chuck for holding down the reticle or wafer or other workpiece, fiducial marks, interferometer mirrors, and other elements such as leveling actuators, not shown as being conventional. Moreover the base structure 18 is more complex than illustrated in FIG. 1, and is typically large and massive including, for instance, a large flat steel or granite structure with a precision machined flat surface 16. The bearings 12 are for instance air bearings or other types of bearings. The stage 10 shown in FIG. 1 and the other figures is capable of movement in only one direction, e.g. the illustrated x axis direction. In other cases, such a stage is also capable of y axis direction motion (into the plane of the figure), and/or in the z axis direction but these cases are not shown for simplicity. A structure as in FIG. 1 can be extended to the case where two independent stages, e.g. for a wafer and reticle, are mounted on the same base structure.
The stage 10 in FIG. 1 has a center of gravity cg (as does any body). The base 18 is mounted on a foundation 22, for instance the floor of a building. Foundation 22 is also referred to as "the ground" since foundation 22 is rigidly coupled to the earth. The base 18 is not rigidly coupled to the foundation 22, in order to isolate the base 18 from any vibration present in the foundation 22. Hence an isolation structure 26 couples the base 18 to the foundation 22. For instance, isolation structure 26 is a set of springs, air jacks, dampers, and/or other types of passive or active vibration dampening devices.
The stage 10 moves as shown in the x direction, back and forth on the base surface 16. This movement is conventionally accomplished by a linear actuator of any type (not shown). The linear actuator is e.g. a magnetic linear motor having one element (the coil) mounted on the stage 10 and a second element (the magnet track) mounted on base 18. The linear actuator can also be a lead screw, ball screw, belt or chain drive, hydraulic device, or any other linear drive device.
Hence a single degree of freedom (x direction) stage 10 is mounted on the base structure 18 which is suspended by isolation structure 26 on the foundation 22, and stage acceleration force F is applied between the stage 10 and the base 18 to move the stage 10 in direction x. Force F causes the stage to accelerate in the x direction and a reaction force -F (equal in magnitude and in the opposite direction to force F) causes the base 18 to move in the opposite direction and can excite flexible vibration modes in the base 18.
The net reaction on the base 18 in FIG. 2 consists of both a reaction force and a reaction moment. The reaction moment is m, equal to the offset distance 1 of the force F from a line passing through the stage center of gravity cg times the force F (m=1.times.F). This reaction moment m is transferred to the base 18 through the bearings 12 supporting the stage 10.
Motion of and vibration in the base 18 causes a relative position error for the stage 10, which requires that the stage 10 be driven forward and backward to maintain a constant position relative to the oscillating base 18; this can reduce the positioning accuracy and increase the settling time. It is of course important in the lithography field that the system not be subject to such vibration forces, in order to preserve the accuracy of the lithography.
Hence it would be desirable to overcome these problems, thereby to reduce settling time and improve lithography accuracy.